Portland cement is a type of hydraulic cement. It is made primarily from the combination of a calcareous material, such as limestone or chalk, and silica and alumina, which are found as clay or shale. These ingredients are ground to a very fine powder, mixed in predetermined proportions and then burned at high temperatures (about 1400 degrees Celsius) to form a clinker. After the formation of the clinker, and subsequent cooling, the clinker is combined with a calcium sulfate material, e.g., gypsum, to avoid flash setting of the cement. The resultant material is then ground to a particle size of between 3,000 to 5,000 cm2/g or finer.
The setting, workability, strength and other performance properties of a hydraulic cement depend in part on the relative ratios of water to cement and the percentage and chemical properties of the additives to the cement. The American Society for Testing and Materials (“ASTM”) defines test procedures for determining the adequacy of hydraulic cement. For example, ASTM c688, published September 2000 and incorporated by reference herein in its entirety, provides test procedures for measuring the adequacy of portland cement when a functional additive is used, such as a setting, retarding, or accelerating additive. After formation of the clinker, the primary material additive is calcium sulfate. Additional functional additives combined with the clinker are typically less than the amount of calcium sulfate added.
A common source of calcium sulfate is gypsum. Both natural gypsum, e.g., as found in sedimentary rock, and synthetic gypsum, e.g., as an industrial waste by-product, are used in the manufacture of portland cement. In its natural state, gypsum is a relatively soft material having a Mohs hardness of 2. When processing natural gypsum during cement production, a significant amount of the gypsum becomes lost due to the difficulty in being able to feed and control fine gypsum together with more coarse gypsum. As a result, special feeders have been designed to accommodate fine and ultra fine materials. These feeders generally have a low storage capacity and require regular and routine attention.
There are also a wide variety of industrial waste by-products that are suitable sources of calcium sulfate. Flue Gas Desulphurization (“FGD”) is one example. FGD is a process that transforms SO2 gas into a primarily sulfur compound. A wet FGD system, and in particular, a forced oxidation wet FGD system has as a principal by-product gypsum which may be used as a source of calcium sulfate. Additional sources of calcium sulfate include any by-product waste with suitable chemistry, such as phosphate and fluoro wastes.
One major difficulty in utilizing a waste by-product is the physical size of the material. Much of the material is colloidal and will carry a high inherent moisture and thus is difficult to feed and control with conventional equipment, such as storage bins and chutes. As a result, manufacturers using waste by-products as calcium sulfate sources have had to resort to special feeding equipment to control flow.
There is a need for improving the feeding and control of calcium sulfate material during the finishing grinding process of portland cement production so as to reduce the percentage of wasted material and without requiring the use of special feeders to control the flow of gypsum.